Here’s something to consider:
First and foremost you are using Category > All Elements of Category to select all Views. That’s not a good idea, probably, since some unexpected things are “views”. Let me give you an example: Project Browser is considered a View in Revit. That’s not something you want to delete. So a better way to select views is using archi-lab.net nodes that let you filter by view type directly like so:
Now, I don’t have a sample file that has View Category parameter so instead I am going to demonstrate the workflow on Sheet Name parameter. Sheet Name is a parameter that would be filled in if given view was placed on a Sheet, otherwise it will be empty. So let’s say my intention here is to delete all views that are not on a Sheet. You can see that in the above image at index . The field is empty.
Now in the above image pay attention to how I set up a filter for Sheet Name, Equals to “”. Then I use the FilterByBoolMask node to filter out Views, not Sheet Names, because the Delete node needs an element not a String.